Soyabean response to rhizobium inoculation across sub-Saharan Africa: Patterns of variation and the role of promiscuity

Article purchased; Published online: 7 Sept 2017Improving bacterial nitrogen fixation in grain legumes is central to sustainable intensification of agriculture in sub-Saharan Africa. In the case of soyabean, two main approaches have been pursued: first, promiscuous varieties were developed to form effective symbiosis with locally abundant nitrogen fixing bacteria. Second, inoculation with elite bacterial strains is being promoted. Analyses of the success of these approaches in tropical smallholder systems are scarce. It is unclear how current promiscuous and non-promiscuous soyabean varieties perform in inoculated and uninoculated fields, and the extent of variation in inoculation response across regions and environmental conditions remains to be determined. We present an analysis of on-farm yields and inoculation responses across ten countries in Sub Saharan Africa, including both promiscuous and non-promiscuous varieties. By combining data from a core set of replicated on-farm trials with that from a large number of farmer-managed try-outs, we study the potential for inoculation to increase yields in both variety types and evaluate the magnitude and variability of response. Average yields were estimated to be 1343 and 1227 kg/ha with and without inoculation respectively. Inoculation response varied widely between trials and locations, with no clear spatial patterns at larger scales and without evidence that this variation could be explained by yield constraints or environmental conditions. On average, specific varieties had similar uninoculated yields, while responding more strongly to inoculation. Side-by side comparisons revealed that stronger responses were observed at sites where promiscuous varieties had superior uninoculated yields, suggesting the availability of compatible, effective bacteria as a yield limiting factor and as a determinant of the magnitude of inoculation response

Effect of Bradyrhizobium japonicum Population on Nodulation, Nitrogen Fixation and Dry Matter Yield on Soybean (Glycine max L Merrill) in Some Ghanaian Soils

In as much as the noted ability of the promiscuous TGx soybean varieties developed by the International Instituteof Tropical Agriculture in Ibadan to nodulate profusely with indigenous bradyrhizobia confers advantages wherelack of fertilizers and suitable inoculants act as disincentives to growing the crop, the ability of these nodules tofix nitrogen should be even more important. Selection for promiscuous nodulators in different soils musttherefore go hand in hand with their nitrogen fixing abilities. The symbiotic performance of promiscuoussoybeans depends upon the population size and survival of bradyrhizobia on the field. A pot experiment wasconducted in the greenhouse within the Geography Department in University of Ghana-Legon using soilscollected from ten farmer’s field located in one Agro Ecological Zone (Coastal savanna) to determine nodulation,nitrogen fixation and dry matter yield by indigenous bradyrhizobia in five Ghanaian soils using threepromiscuous soybean varieties and one American type, Bragg, a non promiscuous variety. Nodulation by thethree promiscuous soybean varieties (Bengbie [TGx 536-2D], TGx 1835-10E, and TGx 1830-20E) rangedbetween 21 to 25 and the non-promiscuous variety (Bragg) recorded 13. Hake soil series which contains thehighest bradyrhizobia cell count (6.0 x 103 cellgsoil-1 also produced the highest number of nodules 22. Hakeagain produced the highest nodule dry weight, (176.0mg), shoot dry weight (3.85g), Total nitrogen (N)(106.0mgN plant-1), Total N fixed (88.9mgN plant-1) and %N fixed (69%). TGx 1830-20E also maintain itssuperiority in nodule dry weight (220.0mg plant-1) production, Total N (105.3mgN plant-1), Total N fixed(88.5mgN plant-1) and % N fixed (83.43). Bragg performed poorly in terms of nodule dry weight (67.0mg plant-1), Total N (83.8mg plant-1), Total N fixed (67.0mg plant-1) and %N fixed (78.05). The Most Probable Number(MPN) counts on sand grown soybean indicated 6000, 5300, 4700, 3600 and 3200 native B. japonicum cells g-1in Hake, Goi, Simpa, Agawtaw and Prampram soils respectively. The nodulation, N content and dry matter yieldresults showed that Ghanaian soils contain effective bradyrhizobia capable of nodulating soybean and N2fixation rendering inoculation unnecessary.Keywords: Agrecological zone. Effective bradyrhizobia. Nitrogen fixation. Nodulation Promiscuous soybea

Influence of Soil Fertility Management Practices on the Nodule-forming Ability of Some Soybean Varieties in a Sub-humid Environment, Nigeria

In a two-year field trials, the influence of eight soil fertility management practices in optimizing the  ability  of six selected promiscuous soybean varieties to form nodules was evaluated in Abakaliki, Southeastern Nigeria which has no history of soybean cultivation in the past. Soil test before planting indicated a pH of 5.50 and 5.85 after harvesting with high available phosphorus, which may have been the reason for its low impact in influencing nodule formation among other practices. Poultry manure and wood ash were both affordable and available low-external inputs which had high potentials in improving and optimizing nodule formation in the soybean varieties to 73.14% and 50.92% in 2008 and 68.55% and 30.77% in 2009, and both are highly recommended as veritable tools for improving soil fertility among the smallholder farmers, where high cost of external inputs has always been the major setback to crop production. Keywords: Inorganic fertilizer, nodule formation, poultry manure, promiscuous soybean, soil fertilit

Breeding for Promiscuous Soybeans at IITA

Peer Revie